1. Field of Invention
The present invention relates to a load driving device controlling a load. More particularly, the present invention relates to a load driving device capable of stably controlling the dimming of the drive of a load by improving a linear response characteristic of a driving current.
2. Description of the Related Art
Conventionally, a load driving device drives a load and is capable of stepwise control of a drive strength. Exemplary loads include a cooling fan, a heater and a backlight. The conventional load driving device employs a Pulse Width Modulation (PWM) dimming method for stepwise control of the drive strength for the load.
A conventional load driving device includes a switching current source supplying a predetermined average driving current to the load by periodically switching a current according to a predetermined switching period. A PWM dimming part outputs a PWM dimming signal for turning on/off the switching current source based on a predetermined dimming step for stepwise control of the drive strength for the load. The dimming step is synchronized by a synchronizing signal of a main circuit part (not shown) of the load driving device, and the switching period has a different period than the synchronizing signal of the main circuit part (not shown).
The change of a load driving current according to the change of a dimming duty ratio in the conventional load driving device will be described with reference to FIG. 1. Herein, the dimming duty ratio refers to a sum of step intervals, Ton of the dimming steps outputting the ON PWM dimming signal within a dimming period Td. There may be a plurality of dimming steps within the dimming period Td. That is, the dimming duty ratio is Ton/Td.
The switching current source of the conventional load driving device, referring to (1-1) of FIG. 1, turns on the switching part (not shown) at t1, t2, t3, t4 according to the switching period T′. The switching current source turns off the switching part if the current supplied to the load reaches a predetermined command value ir. Therefore, a driving current i maintains an average driving current using pulses such as in (1-1) of FIG. 1. The PWM dimming part outputs the PWM dimming signal which turns on/off the switching current source, on the basis of the plurality of dimming steps d1, d2, d3, d4, d5 according to the step interval d′, as shown in (1-1) of FIG. 1. Accordingly, if the PWM dimming signal is interrupted, the switching current source stops to supply the current to the load. Herein, the step interval d′ is different from the switching period T′.
Therefore, the change of the load driving current according to the change of the dimming duty ratio will be described with reference to (1-2) through (1-5) of FIG. 1. As shown therein, the dimming duty ratio becomes larger in order of (1-2)<(1-3)<(1-4)<(1-5).
Referring to (1-2) of FIG. 1, if the PWM dimming part interrupts the PWM dimming signal at the dimming step d1 then the dimming duty ratio is d1/Td. The switching current source turns off the switching part at d1, thereby interrupting the current supplied to the load during the rest of dimming period Td. Therefore, the average driving current i1′ is supplied to the load during the dimming period Td. Referring to (1-3) of FIG. 1, if the PWM dimming part interrupts the PWM dimming signal at the dimming step d2 then the dimming duty ratio is d2/Td. The switching current source turns off the switching part at d2, thereby interrupting the current supplied to the load during the rest of dimming period Td. Therefore, the average driving current i2′ is supplied to the load during the dimming period Td. Referring to (1-4) of FIG. 1, if the PWM dimming part interrupts the PWM dimming signal at the dimming step d3 then the dimming duty ratio is d3/Td. The switching current source turns off the switching part at d3, thereby interrupting the current supplied to the load during the rest of dimming period Td. Therefore, the average driving current i3′ is supplied to the load during the dimming period Td. Referring to (1-5) of FIG. 1, if the PWM dimming part interrupts the PWM dimming signal at the dimming step d4 then the dimming duty ratio is d4/Td. The switching current source turns off the switching part at d4, thereby interrupting the current supplied to the load during the rest of dimming period Td. Therefore, the average driving current i4′ is supplied to the load during the dimming period Td.
Referring to FIG. 1, in the conventional load driving device, the average driving current which is supplied to the load may be not changed linearly. That is, the average driving currents i1′ and i2′ are both the same even though the (1-2) dimming duty ratio d1/Td is different from the (1-3) dimming duty ratio d2/Td. Therefore, in the case of (1-2) and (1-3), the effect on the load will be same even though each dimming duty ratio is different. Further, the average driving currents i3′ and i4′ are both almost the same even though the (1-4) dimming duty ratio d3/Td is different from the (1-5) dimming duty ratio d4/Td. Therefore, in case of (1-4) and (1-5), the effect on the load will be almost same even though each dimming duty ratio is different. The phenomena described above, appears more when the variation of the change of the dimming duty ratio is smaller than a switching off interval within the switching period.
With the conventional load driving device, the average driving current supplied to the load is not changed linearly corresponding to the change of the dimming duty ratio for stepwise control of the drive strength of the load. As such, the conventional load driving device is limited in being able to stably control a driving response of the load according to the dimming duty ratio.
Accordingly, there is a need for an improved load driving device controlling a load that can stably control a driving response of a load according to a dimming duty ratio.